A polishing apparatus provided with a polishing tool, such as a polishing tape or a grinding stone, is used as an apparatus for polishing a peripheral portion of a substrate, such as a wafer. FIG. 14 is a schematic view showing this type of polishing apparatus, As shown in FIG. 14, the polishing apparatus includes a substrate stage 110 for holding a central portion of a wafer W by vacuum suction and rotating the wafer W, and a polishing head 105 for pressing a polishing tool 100 against a peripheral portion of the wafer W. The wafer W is rotated together with the substrate stage 110, and in this state the polishing head 105 presses the polishing tool 100 against the peripheral portion of the wafer W to thereby polish the peripheral portion of the wafer W. A polishing tape or a grinding stone may be used as the polishing tool 100.
As shown in FIG. 15, a width of a portion of the wafer W polished by the polishing tool 100 (which will be hereinafter referred to as a polishing width) is determined by a relative position of the polishing tool 100 with respect to the wafer W. Typically, the polishing width is several millimeters from an outermost peripheral edge of the wafer W, In order to polish the peripheral portion of the wafer W with a constant polishing width, it is necessary to align a center of the wafer W with an axis of the substrate stage 110. Therefore, before the wafer W is placed on the substrate stage 110, centering of the wafer W is performed by holding the wafer W with centering hands 115 as shown in FIG. 16. The centering hands 115 are configured to approach from both sides of the wafer W, which has been transported by a transfer robot (not shown), to contact an edge portion of the wafer W, thereby holding the wafer W. A relative position between the centering hands 115 and the substrate stage 110 is fixed, and the center of the wafer W held by the centering hands 115 is located on the axis of the substrate stage 110.
However, such a conventional centering mechanism has a limit to an accuracy of the wafer centering. As a result, the polishing width may be unstable. Moreover, the centering hands 115 may be worn out, resulting in a lowered accuracy of the wafer centering. Furthermore, when the polishing tool 100 is pressed against the peripheral portion of the wafer W, the wafer W in its entirety is bent, and as a result a defect may occur in the peripheral portion of the wafer W. In order to prevent the wafer W from being bent, a supporting stage (not shown) for supporting a circumferential portion of a lower surface of the wafer W may be provided separately from the substrate stage 110. However, if a substrate supporting surface of the substrate stage 110 is not flush with a substrate supporting surface of the supporting stage, the wafer W is bent.